In JavaScript why is 0 equal to false but when tested by if it is not false by itself

Question

The following shows that  0  is false in Javascript    gt  gt  gt   0     false true   gt  gt  gt  false     0  true   So why does the following print  ha     gt  gt  gt  if   0   console log  ha   ha

User · Answer

Your quotes around the 0 make it a string  which is evaluated as true   Remove the quotes and it should work   if  0  console log  ha

User · Answer

This is the reason why you should whenever possible use strict equality     or strict inequality      quot 100 quot     100 true because this only checks value  not the data type  quot 100 quot      100 false this checks value and data type

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Tables displaying the issue     and      Moral of the story use       table generation credit  https   github com dorey JavaScript-Equality-Table

User · Answer

In JS  quot    quot  sign does not check the type of variable  Therefore   quot 0 quot    0   false  in JS 0   false  and will return true in this case  but if you use  quot     quot  the result will be false  When you use  quot if quot   it will be  quot false quot  in the following case   0  false      null  undefined  NaN     null   undefined  0   false  So if  quot 0 quot     if    quot 0 quot      0   amp  amp    quot 0 quot      false   amp  amp    quot 0 quot       quot  quot    amp  amp    quot 0 quot      null   amp  amp    quot 0 quot      undefined   amp  amp    quot 0 quot      NaN              if true  amp  amp  true  amp  amp  true  amp  amp  true  amp  amp  true  amp  amp  true            if true

User · Answer

The reason is because when you explicitly do  0     false  both sides are being converted to numbers  and then the comparison is performed   When you do  if   0   console log  ha    the string value is being tested  Any non-empty string is true  while an empty string is false      Equal           If the two operands are not of the same type  JavaScript converts the operands then applies strict comparison  If either operand is a number or a boolean  the operands are converted to numbers if possible  else if either operand is a string  the other operand is converted to a string if possible  If both operands are objects  then JavaScript compares internal references which are equal when operands refer to the same object in memory        From Comparison Operators in Mozilla Developer Network

User · Answer

The  if  expression tests for truthiness  while the double-equal tests for type-independent equivalency   A string is always truthy  as others here have pointed out   If the double-equal were testing both of its operands for truthiness and then comparing the results  then you d get the outcome you were intuitively assuming  i e    0     true      true   As Doug Crockford says in his excellent JavaScript  the Good Parts   the rules by which     coerces the types of its operands  are complicated and unmemorable     The lack of transitivity is alarming    It suffices to say that one of the operands is type-coerced to match the other  and that  0  ends up being interpreted as a numeric zero  which is in turn equivalent to false when coerced to boolean  or false is equivalent to zero when coerced to a number

User · Answer

This is because JavaScript uses type coercion in Boolean contexts and your code  if   0      will be coerced to true in boolean contexts   There are other truthy values in Javascript which will be coerced to true in boolean contexts  and thus execute the if block are -  if  true  if      if      if  42  if   0   if   false   if  new Date    if  -42  if  12n  if  3 14  if  -3 14  if  Infinity  if  -Infinity

User · Answer

Equality operator evaluates the arguments after converting them to numbers  So string zero  0  is converted to Number data type and  boolean false is converted to Number 0   So       0     false    true   Same applies to       false     0    true       Strict equality check evaluates the arguments with the original data type       0      false    false  because  0  is a string and false is boolean   Same applies to      false      0     false   In     if  0   console log  ha      The String  0  is not comparing with any arguments  and string is a true value until or unless it is compared with any arguments  It is exactly like      if true  console log  ha      But     if  0  console log  ha       empty console line  because 0 is false

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if  x     coerces x using JavaScript s internal toBoolean  http   es5 github com  x9 2   x    false   coerces both sides using internal toNumber coercion  http   es5 github com  x9 3  or toPrimitive for objects  http   es5 github com  x9 1   For full details see http   javascriptweblog wordpress com 2011 02 07 truth-equality-and-javascript

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It s PHP where the string  0  is falsy  false-when-used-in-boolean-context   In JavaScript  all non-empty strings are truthy   The trick is that    against a boolean doesn t evaluate in a boolean context  it converts to number  and in the case of strings that s done by parsing as decimal  So you get Number 0 instead of the truthiness boolean true   This is a really poor bit of language design and it s one of the reasons we try not to use the unfortunate    operator  Use     instead

User · Answer

I usually do this   x    0     if     x  console log  I am true    else      console log  I am false        Essentially converting string to integer and then boolean

User · Answer

I have same issue  I found a working solution as below  The reason is     if  0  means false  if  -1  or any other number than 0  means true  following value are not truthy  null  undefined  0   quot  quot empty string  false  NaN  never use number type like id as       if  id      for id type with possible value 0  we can not use if  id      because  if  0  will means false  invalid  which we want it means valid as true id number  So for id type  we must use following     if   Id     undefined   amp  amp   Id     null   amp  amp   Id      quot  quot                                                                                                                                                                    else                                                                                                                                                                for other string type  we can use if  string       because null  undefined  empty string all will evaluate at false  which is correct         if  string type variable

User · Answer

It s according to spec    12 5 The if Statement         2  If ToBoolean GetValue exprRef   is true  then  a  Return the result of evaluating the first Statement   3  Else          ToBoolean  according to the spec  is     The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11    And that table says this about strings        The result is  false if the argument is the empty String  its length is zero      otherwise the result is true   Now  to explain why  0     false you should read the equality operator  which states it gets its value from the abstract operation GetValue lref  matches the same for the right-side   Which describes this relevant part as    if IsPropertyReference V   then  a  If HasPrimitiveBase V  is false  then let get be the   Get   internal method of base  otherwise let get be the special   Get   internal method defined below   b  Return the result of calling the get internal method using base as its this value  and passing  GetReferencedName V  for the argument   Or in other words  a string has a primitive base  which calls back the internal get method and ends up looking false   If you want to evaluate things using the GetValue operation use     if you want to evaluate using the ToBoolean  use      also known as the  strict  equality operator

User · Answer

It is all because of the ECMA specs      0     false because of the rules specified here http   ecma262-5 com ELS5 HTML htm Section 11 9 3    And if   0   evaluates to true because of the rules specified here http   ecma262-5 com ELS5 HTML htm Section 12 5

[javascript] In JavaScript, why is "0" equal to false, but when tested by 'if' it is not false by itself?

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